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Hi Mick
For lead acid battery flyback charging follow Warrens Bedini wheel succezs...personally I wouldn't put anything in circuit to suppress the spike.
I know Rick Fredricjh ecommends a neon bulb of certain voltage limiting purpose to save the mosfeys in the stuff he sells but my meager opinion is use brush commutator for least resistance plus the destructive transient spikes now not such a big worry.
Just as long as you always have the spike gathering reservoir hooked up to flyback steering diode, that destructive transient is going to hit the battery and not go back through mosfet or transistor and fry it...so never operate if solidstatte without the charging baytery....as for csps.you will have problem whenever you disconnect cap from flyback diode and coil to discharge it to load (battery) non reflectivrto input, so solution for that is to have two steering diodes and two collector-caps, discharge cap A while cap B fills and vice versa....now you will be doing a sort of AC output if you choose to flip polarity each cap discharges.
Anyways my meager advice for charging lead acid 12vbatteriez with cap discharge is knock cap into battery at one second intervals, discharge cap when it is at 36v, and let cap drop down to 23v no less than that...cap discharge to charge is not same as flyback charge perhaps you can make cap discharge style work better in that it uses less energy to fill the caps and charhes battery faster and better but flyback charge into lead acids sure is simple..
For rotovertor motor charging via run cap phase, if running RV on 120vac and you have similar voltage across your run cap, put a FWBR AC legs across that run cap, then DC side straight into lead acid battery...more in parallel the better this very dangerous the DC you can't "shake off" like AC if you in process of being Lee trocuted, also if you disconnect battery on DC side of FWBR there is good chance of bad shock e en without touching anything...just fingers near will give you a jolt...so always first disconnect FWBR on AC side first, then disconnect battery.
Hector likes to run a step down transformer in series off the run cap phase to charge batteries at 14.5v or so
Lately he parralled this charge into batteries with HVDC too in that rotovertor battery charging loop circuit he has put up 20 times this year at least...so 12 battery powed an invertor, invertor runs rotovertor motor at 120vac, then series step down transformer into fwbr back into battery to charve... Then also Line 2 of rotovertor gets the HVDC fwbr treatment and this slams into same battery.
Kone